Heretofore, as a catalyst for converting automotive exhaust gas, three-way catalysts have been used, three-way catalysts which convert exhaust gases by carrying out the oxidation of carbon monoxide (CO) and hydrocarbons (HC) in the exhaust gases and the reduction of nitrogen oxides (NOx) in them simultaneously under the driving condition of theoretical air-fuel ratio (or being stoichiometric). As for such a three-way catalyst, the following have been known widely: those in which a porous support layer comprising γ-alumina, and the like, is formed on a heat-resistant substrate comprising cordierite, and so forth, and then a catalytic metal, such as platinum (Pt) or rhodium (Rh), is supported on the resulting porous support layer, for instance.
Meanwhile, from the viewpoint of global environmental protection, carbon dioxide (CO2) in exhaust gases that are emitted from internal combustion engines for automobile, and the like, has been considered a problem recently, and the so-called “lean-burn” in which they undergo lean burning in oxygen-rich atmosphere is regarded promising as a measure for solving it. In this lean-burn, it is possible to suppress the occurrence of CO2, namely, its combusted exhaust gas, because it is possible to reduce the consumption amount of fuel.
By the way, the performance of exhaust-gas converting catalyst depends greatly on the set-up air-fuel ratio (A/F) of engine. Specifically, on the lean side where the air-fuel ratio is great, the oxidation action becomes active and the reduction action becomes inactive because the oxygen amount in after-combustion exhaust gas becomes abundant. Accordingly, in conventional three-way catalysts which can convert CO, HC and NOx in exhaust gas efficiently at the theoretical air-fuel ratio (or at stoichiometry), no sufficient converting performance is exhibited with respect to the removal of NOx by means of reduction in lean atmosphere that turns into being oxygen rich. Consequently, an exhaust-gas converting catalyst for lean-burn engine has been desired, exhaust-gas converting catalyst which can convert NO efficiently even in oxygen-rich atmosphere.
As for such an exhaust-gas converting catalyst for lean-burn engine, an NOx storage-and-reduction type catalyst has been put into practical use, NOx storage-and-reduction type catalyst in which an NOx storage material comprising an alkaline-earth metal such as barium (Ba), and the like, and a noble metal such as Pt are supported on a porous supporter. This NOx storage-and-reduction type catalyst for lean-burn differs from the three-way catalysts in that, even when being oxygen-rich exhaust gases, it stores and then reduces NOx efficiently to convert them.
However, SO2 is included in exhaust gases, SO2 which generates because sulfur (S) being included in fuels is combusted. This SO2 is turned into SO3 by being oxidized by means of noble metal in high-temperature exhaust gases. And, this SO3 turns into sulfuric acid due to water vapor being included in the exhaust gases. When SO3 and sulfuric acid thus generate in the exhaust gases, sulfites and sulfates are generated by means of the reactions between the resulting SO3 or sulfuric acid and the NOx storage material, and then it became apparent that the NOx storage material is poisoned by means of this. When the NOx storage material is thus deteriorated by means of S poisoning, the after-durability NOx converting performance has lowered because it is no longer able to store NOx.
Hence, an exhaust-gas converting apparatus has been known, exhaust-gas converting apparatus in which an S storage catalyst, which stores S components from exhaust gases, is disposed at the previous stage to an NOx storage-and-reduction type catalyst (See Patent Literature No. 1, for instance).
The S storage catalyst in this exhaust-gas converting apparatus is completed by supporting an S storage material, which comprises an alkali metal, such as potassium or sodium, or an alkaline-earth metal, such as calcium or barium, and a catalytic noble metal, such as platinum or palladium, onto a supporter, which comprises alumina. In accordance with such an S storage catalyst, since it is possible to store S components in exhaust gases, it is possible to suppress the S poisoning of the NOx storage-and-reduction type catalyst by disposing this S storage catalyst at the previous stage to the NOx storage-and-reduction type catalyst.
Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2006-144,624